Study of the basis of partially selective effects of folate analogues on leukemic cells. In this research the effect of methotrexate and other folate analogues on dihydrofolate reductase, on the synthesis of DNA by de novo and salvage pathways, and on cell morphology and cell kinetics will be examined: (a) in leukocytes from all types of leukemic patients (pretreatment), (b) in bone marrow suspensions obtained from normal subjects, (c) in cultured HeLa, J111 or other human cell lines. (d) in normal human intestinal mucosa. The data obtained thus, as well as that from studies on uptake of analogues by cells will be examined for evidence of the bases of selectivity, and for indications of the analogue having the most selective action. Study of the immunosuppressive effect of folate analogues. The effects of analogues on cultured human lymphocytes that are responding to antigenic stimuli wll be examined for indications of the mechanism of immunosuppression, to determine which analogue is most effective at levels causing minimum systemic toxicity, and whether other immunosuppressive drugs act synergistically with folate analogues. The most promising analogues will be tested for suppression of graft-versus- host disease in two or more in vivo mouse models, and effects of other agents, dose, regimen, etc. on the outcome will be examined.